In semiconductor microlithography a critical operation is the alignment of patterns on a photomask to previously-printed images on a circuit substrate, such as a substrate of silicon, ceramic, or other material. This alignment is typically made to a tolerance approaching, and often exceeding, one micrometer (10.sup.-6 meters). In general, two images are observed using a microscope, and either the photomask or the substrate is manipulated into alignment with the other.
One problem that arises for the case of proximity lithography is that the photomask and the substrate must not be in contact with each other during alignment. If contact were made, the relative sliding motion between the photomask and the substrate could damage the very fine geometric features.
More particularly, due to an inter-surface gap which must be maintained during alignment, the features to be observed on the photomask and those on the substrate cannot both be simultaneously in sharp focus under the microscope. As a result, this often requires the use of a lower-magnification microscope objective, having a desired higher depth-of-field, than is optimum for the alignment tolerances sought.
Several techniques for solving this problem are known to the inventor, and include the following.
A first technique simply accepts the poorer alignment accuracy resulting from the use of the lower-magnification microscope objective, and adjusts the circuit design and fabrication process to accommodate the resulting overlay errors.
A second technique employs a revolving-turret nosepiece on the microscope and installs both a high-magnification, low depth-of-field objective and a low-magnification, high depth-of-field objective. The latter objective is used during alignment; then, just prior to the exposure, when the gap between the photomask and the substrate is typically reduced to contact (or near-contact) to achieve the best possible printing fidelity, the former objective is employed to check the alignment. If an alignment error is detected, the gap is increased again to the alignment condition, and the process is repeated. This process is often repeated many times, and may require in excess of 30 minutes to achieve the desired overlay accuracy. A further disadvantage of this technique is that each time the substrate is brought into contact with the photomask, there exists the potential to cause damage to either the substrate or the photomask.
A third technique employs an automatic alignment system, which might be one of two types. A first type is coherent-illumination based, and requires the use of a laser to detect and locate features with high accuracy over a large depth-of-field. A second type is image processing based and uses a video frame grabber and image processing computer to identify and accurately locate the target images on the photomask and substrate. This system requires a microscope which is capable of quickly and accurately auto-focusing under computer control. Systems embodying one or more of these features are known as an AL2000 and an ALX, both of which are manufactured by Karl Suss America, Inc. of Waterbury Center, Vt.
It is an object of this invention to provide a method and apparatus to accurately bring two planar surfaces into a predetermined spatial relationship with one another.
It is a further object of this invention to provide a method and apparatus for accurately and quickly aligning a photomask to a surface of a substrate.